Observed behaviour

isSpellContainer checks for a stack size of 1
Things like Sophisticated Backpacks allow you to stack items with the same NBT though

NBT itself is valid:

{
    ISB_Spells: {
        data: [
            {
                id: "irons_spellbooks:oakskin",index: 0,level: 2,locked: 1b
            }
        ],
        maxSpells: 1,
        mustEquip: 0b,
        spellWheel: 0b
    }
}

Expected behaviour

N/A

Steps to reproduce

N/A

Server Type

Dedicated Server

Crashlog

No response

Iron's Spells N Spellbooks version

1.19.2 - 3.0.1

Forge version

1.19.2 - 4.3.0

Other mods

No response

fixed in 3.1.0